1. Field of the Invention
The present invention relates generally to recycled exhaust gas cooling systems for natural gas fueled internal combustion engines; and, more particularly, to systems for controlling temperature and condensate in cooled exhaust gas for recycle to natural gas fueled internal combustion engines to effect engine operating characteristics and emission control.
2. Description of Related Art
Electric energy generation in this country has lagged behind demand. There are a number of reasons for this, but chief among them is failure of traditional energy producers to replace spent units and capitalize new plants. This has been, in part, due to increased air quality regulations. In addition, new challenges face electric generation-security. Events of Sep. 11, 2001 showed this nation its vulnerability to terrorist attack. Vital operations, such as police, medical and civil defense that relied upon the electric power “grid” for service, realized that their needs were susceptible to disruption and viewed stand-alone unit alternatives as well as micro grids as a possible solution. These alternatives, however, have their own problems.
One reason for the growth in demand is the increased use of computers and other technology for industrial and business purposes, as well as personal use. As computer usage continues to grow, the use of power-consuming peripheral technologies, such as printers, cameras, copiers, photo processors, servers, and the like, keep pace and even expand. As business use of computer based equipment continues to rise, as do the number of in-house data servers, outsourced data storage facilities, financial systems, and Internet-related companies requiring constant electrical uptime and somewhat reducing traditional peak demand times, requirement for reliable, cheap, environmentally compliant electrical power continues to grow.
Other technological advances have also increased electrical energy demand. Increased use of power consuming devices in every aspect of life from medical to industrial manufacturing robots, as well as innovations in almost every research and industrial field are supported by increasingly complex technology, which requires more electrical power to function. CAT scans, NMRs, side looking X-rays, MRIs and the like all take electrical power.
As a result, the Federal Government deregulated power generation, and a number of states have begun to establish competitive retail energy markets. Unfortunately, the deregulation process has not provided adequate incentives for industry entities to construct generating facilities, upgrade the transmission grid, or provide consumers with price signals to enable intelligent demand-side management of energy consumption. With the deregulation in the utility market, energy (kWh) has become a commodity item that can be bought or sold. However, swings in supply and demand leave end users open to fluctuations in the cost of electricity.
According to the ETA, to meet projected increases in demand over the next 20 years, at least 393 GW of additional generating capacity must be added. In some areas, the growth in demand is much higher than the projected two percent average (e.g., California's peak electricity demand grew by 18 percent between 1993 and 1999, while generating capacity increased by only 0.3 percent.) Despite California's highly publicized energy situation, a similar problem exists for other states as well; the New York Independent System Operator recently stated that 8600 MW of additional generating capacity (a 25 percent increase) must be added by 2005 to avoid widespread shortages that may lead to blackouts.
In addition to the mismatch between demand and generating capacity, the physical transmission infrastructure necessary to deliver power from geographically remote generating facilities to the consumer's location is unable to support the increased load. Even under today's operating conditions, the transmission grid is subject to stress and occasional failure.
Additionally, security and reliability of source has become of increasing concern. Vulnerability of grid systems and blackouts has become more commonplace. Strategic industries are looking to cut energy costs, increase reliability, and assure security. This has lead to an interest in distributed market technologies. The potential market for distributed generation has become vast without adequate means for fulfilling this need. Again, inefficiency, reliability, and environmental concerns are major barriers. The compelling economics are made on engine efficiency without the financial benefit of waste heat usage, yet with all of the same customer reluctance to accept hassles. Industry estimates indicate that the existing market for distributed generation is $300 billion in the United States and $800 billion worldwide.
The need to leverage existing technology while transitioning to alternative energy sources is an important driver for meeting this challenge. Although most existing distributed generation sites use small gas turbine or reciprocating engines for generation, there are many alternatives that are being considered over the longer term. Technologies, such as micro turbines, are currently available, but only used at a relatively small number of sites. These newer generators offer some inherent advantages, including built-in communications capabilities. It is anticipated that fuel cells will be available in the next five years, which will provide some highly appealing, environmentally friendly options.
As it stands today however, small gas turbine and reciprocating engines comprise a substantial proportion of existing generator technology in the market and will for some time to come for a number of reasons. Engines provide the best conversion efficiency (40%), and they can operate using non-pressurized gas. Micro turbines, on the other hand, require compressed gas and conversion efficiency is lower (approximately 30%). These latter generators tend to be used in wastewater, landfill, and other specialty sites, where a conventional prime mover is unable to stand up to poor fuel quality. Therefore, for utilities to truly benefit from a distributed generation scheme over the short term, they must look to the existing generator technology to provide a sustainable and affordable solution.
Waste heat utilization or co-generation is one way to meet this challenge. In the case of power generation, the waste heat is not used, and the economics are based largely on the cost of the electricity produced (i.e. heat rate is paramount), with little consideration for improved reliability or independence from the electric grid. The anticipated fluctuation in energy costs, reduced reliability, and increasing demand has led end users to consider maximizing efficiency through use of heat from generation of on-site generating-heat capture systems, i.e. co-generation, or “Combined Heating and Power” (CHP).
Co-generation of electricity and client process/utility service heat to provide space heating and/or hot water from the same unit is one solution. Co-generation provides both electricity and usable process or utility heat from the formerly wasted energy inherent in the electricity generating process. With co-generation, two problems are solved for the price of one. In either case, the electricity generation must meet stringent local air quality standards, which are typically much tougher than EPA (nation wide) standards.
On-site co-generation represents a potentially valuable resource for utilities by way of distributed generation. A utility can increase capacity by turning to a “host” site (e.g. industrial user) with an existing generator, and allow them to parallel with the grid and use their generator capacity to handle peak volumes. From the utility's point of view, the key advantages to a distributed generation solution are twofold: improved system reliability and quality; and the ability to defer capital costs for a new transformer station.
For customers who can use the process/utility waste heat, the economics of co-generation are compelling. The impediment to widespread use is reliability, convenience, and trouble-free operation. Co-generation products empower industrial and commercial entities to provide their own energy supply, thus meeting their demand requirements without relying on an increasingly inadequate public supply and infrastructure. Unfortunately, to date, the most widespread and cost-effective technologies for producing distributed generation and heat require burning hydrocarbon-based fuel. Other generating technologies are in use, including nuclear and hydroelectric energy, as well as alternative technologies such as solar, wind, and geothermal energy. However, burning hydrocarbon-based fuel remains the primary method of producing electricity. Unfortunately, the emissions associated with burning hydrocarbon fuels are generally considered damaging to the environment and the Environmental Protection Agency has consistently tightened emissions standards for new power plants. Green house gases, as well as entrained and other combustion product pollutants, are environmental challenges faced by hydrocarbon-based units.
Of the fossil fuels, natural gas is the least environmentally harmful. Most natural gas is primarily composed of methane and combinations of Carbon Dioxide, Nitrogen, Ethane, Propane, Iso-Butane, N-Butane, Iso-Pentane, N-Pentane, and Hexanes Plus. Natural gas has an extremely high octane number, approximately 130, thus allowing higher compression ratios and broad flammability limits. Natural gas is the most popular fuel choice for engine cogeneration because it is relatively clean, already widely distributed, safe, and it provides favorable engine power and durability. However, many of the markets that would be best served by the economics of engine-based co-generation have such poor air quality that strict exhaust emission limits have been instituted by air quality regulating agencies. The exhaust emissions limits on oxides of nitrogen, carbon monoxide, and non-methane hydrocarbons are so restrictive that no technology exists to allow raw exhaust emissions from any engine operating on any hydrocarbon fuel to enter the atmosphere without exhaust aftertreatment and a variety of other strategies. problem area is the emissions produced by these natural gas engines. Never-the-less, natural gas fueled engines provide a valuable power source for distributed generation.
Internal combustion engines utilized for combined heat and power are designed so that heat generated during combustion can be recovered from the engine coolant and exhaust and then transferred to a cogeneration client. Prior art co-generation systems have had to comply with strict emissions limits by either altering the air/fuel ratio from an excess-air strategy to a stoichiometric strategy to facilitate the successful operation of non-selective three-way catalysts or by applying selective catalytic reduction (SCR) exhaust aftertreatment technologies to the exhausts of excess-air fueled engines. Each approach has undesirable consequences compared to the original excess-air, or lean-burn, operation. The stoichiometric air/fuel ratio with three-way catalyst strategy increases combustion temperatures to such an extent that the engine must be derated to control detonation and mitigate accelerated wear. This scenario also results in reduced fuel efficiency compared to a lean-burn engine. The SCR emissions compliance approach allows a lean-burn engine to operate at full load with excellent fuel efficiency, but at the expense of having to store chemicals on site and then inject them in a very controlled fashion into the lean-burn exhaust stream. After injection the exhaust becomes compatible with catalytic emissions reduction techniques.
Thus, applying SCR technology is expensive, complicated, and generally not considered an option for engines producing less than one megawatt of electric power. Prior art co-generation systems employing internal combustion engines, and specifically, natural gas fueled engines, have suffered from the myriad of problems including elevated head temperatures and inability to deliver large quantities of process and/or utility heat not true to the co-generation client. Excessive head temperatures lead to inefficient operation and unacceptable environmental conditions, which include excessive use of fuel as well as significant thermal NOx production.
It is well known that emission reduction for natural gas engines can be accomplished by recycling of exhaust gases to make the engines “run lean.” This is the same term used to describe the diluted air/fuel condition that exists when excess-air is introduced into combustion. This method of combustion combines the positive cool combustions and high fuel efficiency attributes of operating with excess air with the compatibility between simple non-selective three-way catalysts and exhaust that stoichiometric engines benefit from without incurring the need to derate the output of the engine. This method of emission control has decided advantages over exhaust gas after treatment such as a catalyst. When catalysts are still required they are minimized.
For this reason, numerous systems have been devised to recycle exhaust gas into the fuel-air induction system of an internal combustion engine for the purposes of pre-heating the air-fuel mixture to facilitate its complete combustion in the combustion zone, for re-using the unignited or partially burned portions of the fuel which would otherwise pass to exhaust and into the atmosphere, and for reducing the oxides of nitrogen emitted from the exhaust system into the atmosphere. It has been found that approximately 15% to 20% exhaust gas recycling is required at moderate engine loads to substantially reduce the nitrogen oxide content of the exhaust gases discharged in the atmosphere, that is, to below about 1,000 parts per million.
Although the prior art systems have had the desired effect of reducing nitrogen oxides in the exhaust by reducing the maximum combustion temperature as a consequence of diluting the fuel-air mixture with recycled exhaust gases during certain operating conditions of the engine, these systems have not been experienced broad market application in the field of cogeneration where continuously high engine output is the normal mode of operation and the level technological implementation on engine controls often lacks the automotive market.
The formation rate of nitrogen oxide emission is a direct function of combustion temperature. Any incremental increase in rate of cooled EGR applied during combustion at any load results in lower peak combustion temperatures and hence lower untreated NOx emissions. The propensity for detonation, another temperature dependent phenomenon, is also reduced for each incremental increase in cooled EGR. EGR rates from 20–25% are generally required to achieve similar detonation control characteristics and raw engine-out NOx formation rates as compared to high excess-air strategies.
Thus, natural gas engine survivability with regards to detonation at high load is largely dependent on the success of appropriately metering and cooling the recirculated exhaust gas. While there are numerous products available commercially for the accurate control and metering of the amount of excess-air used in traditional lean-burn (lambda>1) engines, the same is not true for recirculated exhaust gas metering and cooling in non-automotive applications. The industrial nature of the cogeneration market does not benefit from the research and development budgets of the mass produced automotive products and thus the application of advanced emissions control techniques such as cooled EGR has thus far been limited. One challenges for applying EGR highly loaded natural gas engines includes is to provide sufficient cooling of the recirculated exhaust gas such that impacts on volumetric efficiency of air induction are minimized. The higher the temperature of the recirculated exhaust gas as it enters the air/fuel stream, the more difficult it becomes to induce adequate air flow to support full load combustion. Furthermore, the higher the EGR temperature, the higher the compressed intake charges temperature from the turbocharger both before and after the charge-air intercooler. The higher the EGR temperature induced into the air stream, the more this higher temperature offsets the benefits of EGR with regards to detonation mitigation.
Natural gas fired, internal combustion driven, co-generation systems have previously suffered from one or more disadvantages. Specifically, the EGR system did not recycle exhaust gas to the intake engine manifold at sufficiently low temperature to foster low cylinder head temperatures. Simultaneously, turbo charged fuel systems, because of the compression, increase intake fuel manifold temperatures causing the same effect. This combination of disadvantages made natural gas fueled, internal combustion driven co-generation systems a marginal candidate for client based distributed generation complexes.
Additionally, since combustion of natural gas (methane) produces large amounts of water, this water is entrained as vapor in the exhaust gas. Although reducing the temperature of the exhaust gas allows more volume of gas to be introduced into the intake gas in the manifold, it can also present a water vapor saturated exhaust gas mixture, which precipitates at the carburetion site. Thus, cooling of the gas in a non-regulated manner can present condensation problems in the intake venturi. If the gas is cooled in the EGR cooler, so that the vapor pressure in the exhaust gas approaches saturation, further cooling by introduction of air and fuel causes an immediate condensation within the intake manifold, which can lead to erratic engine performance, corrosion, and the like.
In addition, the presence of large amounts of water vapor in the recycled exhaust gas displaces other combustion gases in the intake manifold making the engine unstable. Problematically, introducing the recycle gas at a temperature above the dew point (to prevent induction condensation) leads to detonation and NOx production causing the engine to burn out of NOx emission ranges as well as shorting engine life. Contrawise, some water vapor in the recycled exhaust gas is beneficial to the engine performance.
The previous solution was to increase the capacity of the primary cooler. However, in addition to being hard to “tune”, large primary coolers are expensive because they must handle very high temperatures and corrosive media. Because the volume of very hot gas varies as the engine accelerates or decelerates, the water vapor in the exhaust gas exiting such primary coolers, is difficult to regulate. When the cooler is a single unit, it is difficult to incrementally vary the composition of the output gas.
Typical prior art exchangers are gas-to-liquid, not gas-to-air. These heat exchangers are found commercially on heavy-duty truck and bus engines for the purpose of cooling re-circulated exhaust gas for diesel emissions compliance. However, the dynamics of these systems are different. For all the reasons mentioned above, these engines have different operating characteristics than natural gas fueled engines. First, the engine's coolant is typically the cooling medium in order to achieve exhaust gas cooling in a minimum amount of space. This inherently limits the minimum EGR temperature that can be achieved to well over 200° F. even with the best of liquid-to-gas heat exchangers. For diesel engines, this does not pose a serious threat to engine life because detonation is not a concern with compression ignition engines. Second, gas and diesel engine, liquid-to-gas, EGR cooling heat exchangers are typically of stainless steel construction in flat-plate and tube-in-shell configurations. This design is very expensive, but necessary to handle the exhaust gas heat.
In addition, certain spark ignited engines, such as gasoline and natural gas fueled engines, have different EGR constraints than compression ignition engines, such as diesel. In automotive gasoline engines, EGR is a prevalent emissions reduction technique. However, gasoline engine using EGR emission control do not require dedicated EGR cooling because: 1) the EGR rate is usually limited to 10–12% minimizing the impact on charge-air temperatures; and; 2) the EGR is not employed at high load where temperature-influenced ignition is of most concern.
It would, therefore, be advantageous to have an air-to-gas cooled system wherein recycled exhaust gas temperature and vapor pressure could be regulated to maintain engine performance and emission compliance through out the load range while preventing inadvertent condensation within the intake manifold which causes sporadic engine performance and reduces engine life. It would be further advantageous to have an air-to-gas cooled system which allows the natural gas fired internal combustion engine using EGR to operate at higher shaft power output without inducing detonation; allows the engine to operate with better fuel efficiency due to increased detonation tolerance when advancing ignition timing; and, allows the engine to operate at the same maximum continuous power output. In addition, it would be advantageous to have an EGR cooling system which could be “tuned” to facilitate using recycled exhaust gas for a lean burning natural gas fired engine which results in not only a lean burn but also reduced head temperatures leading to reduces thermal emissions and greater efficiency without unwanted detonations and/or water condensation within the intake manifold.